(1) Field of the Invention
The present invention relates to an alkaline eluting solution for use when preparing a lithographic printing plate by forming a toner image by the electrophotography and eluting a non-image area by an alkaline solution, and a process for preparing a lithographic printing plate by using this alkaline eluting solution.
(2) Description of the Related Art
Among known conventional lithographic printing plate material, are a PS (presensitized plate) prepared by using a photosensitive resin and forming an image by a photochemical reaction or the like, a silver master formed by using a silver halide photosensitive material, and an electrophotographic master paper formed by using a photoconductive material such as zinc oxide.
A diazo type photodecomposable polymer, vinyl type photopolymerizable polymer or photocrosslinkable prepolymer comprising a photosensitive resin is used as a material of a lithographic printing plate, and the characteristics of the photosensitive resin are utilized. The printing plate formed by using the above polymer has an excellent printing resistance but a low sensitivity, and thus a block copy film and an ultraviolet ray curing apparatus must be used at the plate-forming step. Therefore, the printing plate material is unsatisfactory in that the plate-forming step is complicated and the manufacturing cost is high.
The lithographic printing plate material formed by the diffusion process using a silver halide has an excellent sensitivity and resolving power, but the plate material is expensive and has an unsatisfactory poor printing resistance.
It is known that the lithographic printing plate material formed by the electrophotography using a photoconductive material can be formed into a lithographic printing plate by forming an image on the surface of the plate material by reflected rays from an original. According to an example of this image-forming process using zinc oxide, a dispersed mixture of zinc oxide (pigment), a binder (resin) and a sensitizing dye is uniformly coated (to a thickness of 10 to 15 .mu.m) on the surface of a substrate having a low electric resistance, such as a paper or metal laminate, or an aluminum plate, to thereby form an electrophotographic photosensitive plate. A toner image is formed on this plate by rays reflected from an original by the electrophotography, and the surface of the formed photosensitive plate is then subjected to an etching treatment (desensitizing treatment) to desensitize the non-image areas, and thus a printing plate that can be set to an offset printing machine is provided. Nevertheless, the application of this zinc oxide printing plate as a lithographic printing plate, for which a high resolving power and printing resistance are required, is restricted because the surface particles are coarse or the adhesion of the photosensitive layer to the support is poor. Moreover, the surface of the photosensitive plate is not sufficiently desensitized and the application range is narrowed when certain kinds of inks are used.
According to another example of the process for preparing a lithographic printing plate by the electrophotography, a dispersed mixture is formed from an organic photoconductive pigment and a binder (resin) and an image is formed by using this photosensitive material in the same manner as described above with respect to the zinc oxide photosensitive plate. An alkali-soluble resin is used as the binder (resin), and the photosensitive material is coated on the surface of a substrate subjected to a hydrophilic treatment or on the surface of an aluminum plate subjected to a hydrophilic treatment by sandblasting or alumitizing, to form a photosensitive plate. A toner image is formed by electrophotography using a thermosetting or thermoplastic toner for the liquid developer. The photosensitive layer in the non-image area is eluted by using an alkaline eluting solution, while leaving the toner image area, to expose the hydrophilically treated surface or the sandblasted or alumitized surface of the aluminum plate, and finally, a lithographic offset printing plate is obtained. The printing plate material prepared from the foregoing starting materials shows a high resolving power, due to the fine particles of the liquid toner, and a high printing resistance and a high resolving power due to the high water-resistance and high mechanical strength of the aluminum plate. As the alkaline eluting solution, there can be mentioned, for example, and alkaline eluting solution containing sodium silicate, sodium hydroxide or sodium carbonate as the main component, an alcoholic eluting solution comprising benzyl alcohol or propanol as the main component, and a mixture thereof. An appropriate eluting solution is selected according to the kind of the binder resin or toner. A positive type eluting solution for a PS plate, customarily used in this field, can be used as an example of the commercially available alkaline eluting solution. When toner images were formed by using organic photoconductive photosensitive materials prepared by the present inventors, and were eluted with these known eluting solutions, the quality of the plate images after the elution depended greatly on the immersion time and the change of the temperature of the eluting solution, and good images were difficult to obtained.